1. Field of the Invention
This invention relates to an improved apparatus for use in wellbore telemetry operations. More particularly, this invention relates to an improved cable system for obtaining real-time information about the drilling process and the formations being drilled, which real-time information is measured while drilling (MWD) and transmitted to the surface immediately at a rate high enough to support high data transmission rates such as video or televiewer systems.
2. Description of Prior Art
In the oil and gas industry, in particular, there is a great need for real-time information about the drilling process and the formations being drilled. Ideally, the information would be measured while drilling and transmitted to the surface immediately at a rate high enough to support video or televiewer systems. However, current data transmission rates using conventional technology are on the order of 1 to 10 bits per second which, nevertheless, generates a substantial amount of revenue for the measurement-while-drilling business. By increasing the data rates into the megahertz range, not only would there be significant economic implications, but such high data rates would enable the real-time use of virtually any instrumentation to observe the drilling process and surrounding formations.
A number of efforts have been made to solve the transmission rate problem for measurement-while-drilling systems. Even the obvious solution of connecting an electrical or fiber optic cable to the instrumentation package has been attempted. The difficulty with the obvious solution lies in arranging to thread and retrieve cable through thousands of feet of drill pipe under operating conditions. This becomes a very large logistics and material handling problem if standard cable is used. A cable guaranteed to survive and to be reusable is quite bulky. It must be strung through all of the pipe to be used before the drill string is assembled, or alternatively, connectors must be used at each end of each stand of pipe. This drastically reduces the operation speed and, thus, entails large costs for drilling rig time. Indeed, the difficulties are so severe that this approach is almost never used.
An additional problem associated with conventional wellbore telemetry systems is the reliability of the means for transmitting the information between the subsurface region of the wellbore and the surface locations around the wellbore. In particular, in rotary drilling, a borehole is advanced by rotating a drill string equipped with a drill bit. Sections of drill pipe, typically 30 feet in length, are added individually to the drill string as the borehole is advanced. It will be apparent to those skilled in the art that cabling for transmitting a signal between the subsurface and surface locations of a wellbore must be such as to permit the addition of individual pipe sections to the drill string. One early approach to this problem involved the use of a continuous cable adapted to be lowered inside the drill string and to make contact with a subsurface instrument. This technique, however, required withdrawing the cable each time a pipe section was added to the drill string.
More recent approaches have involved the use of special drill pipe equipped with data conductors. Each pipe section is provided with connectors that mate with connectors of an adjacent pipe section so as to provide a data transmission conduit across the joint. Disadvantages of this system include the need for special pipe sections and the difficulty of maintaining insulation of the electrical connectors at pipe section joints.
U.S. Pat. No. 3,904,840 teaches an apparatus having coiled conductors stored therein for use in a wellbore telemetry system. The apparatus includes a tubular container, an insulated electric conductor mounted in the container in a configuration which includes left-hand and right-hand coils, and means for dispensing the conductor from opposite ends of the container. The apparatus permits the conductor string to be lengthened as the drill string is lengthened.
U.S. Pat. No. 4,181,184 teaches a soft-wire conductor wellbore telemetry system in which a resilient conductor having an outer flexible insulating coating and an inner flexible conducting core is employed in a drill string to maintain an electric circuit between a subsurface and a surface location. The conductor is inserted into the drill string in a generally free-hanging, random fashion to store excess length of conductor which is utilized as the drill string is lengthened. The stored conductor is maintained in the drill string in a generally untangled state due to its kink-resistant mechanical and physical properties. In addition, the frictional drag of the flowing drilling fluid tends to straighten and disentangle the conductor.
U.S. Pat. No. 4,534,424 teaches a retrievable telemetry system for installing and retaining a conductor between a surface terminal and a subsurface location in a drill string in which one end of the conductor is lowered into the drill string and is anchored to the drill string of a subsurface location. The upper end of the conductor is taken in from the surface until the conductor is tensioned to a selected amount. The upper end of the conductor is then conducted to the surface terminal. As each drill pipe section is added to the drill string to advance the depth of the well, the tension of the conductor is controlled to reduce fatigue failure of the conductor. In accordance with one disclosed embodiment, the tension of the conductor is controlled by connecting a conductor section of a selected length between the surface terminal and the upper end of the conductor.